Selective herbicides are routinely applied to control weeds among crop plants. The weeds would otherwise compete for available nutrients, water, and light, and thus reduce crop yield and quality. Selective herbicides which show low toxicity to crop species, while playing an important role in the control of weeds in modern agriculture, are often available for only the major crop species because of the high cost of development.
An alternative to the identification of new selective herbicides for use with particular crop species is the genetic modification of susceptible crop species so that they are resistant to non-selective herbicides. One method of achieving this is through the genetic transformation of plants to herbicide resistance. The prerequisites for such an approach are the ability to transform the species of interest, and availability of a gene which confers resistance to the herbicide of interest.
Resistance to a specific herbicide may be a result of introduction into a plant of a gene conferring resistance to the herbicide or may be as a result of long periods of exposure to the herbicide. The resistance may be the result of changes in enzymes which are involved in particular biosynthetic pathways. For example, the broad spectrum weed killer glyphosate (phosphonomethylglycine) acts by inhibiting the enzyme 5-enolpyruvyl-3-phosphoshikimate synthetase that converts phosphoenolpyruvate and 3-phosphoshikimaic acid to 5-enolpyruvate-3-phosphoshikimaic acid in the shikimic acid pathway in bacteria. Following mutagenesis of Salmonella typhimurium, an altered synthetase enzyme resistant to glyphosate has been identified and introduced into plants where it confers resistance to glyphosate.
It is of interest to identify other biosynthetic pathways which may be affected by specific herbicides as a means of developing herbicide resistant plants. Two unrelated classes of herbicides, the sulfonylureas and the imidazolinones, notable for their high herbicidal potencies and low mammalian toxicities, target the enzyme acetolactate synthase, the first common step in the biosynthesis of the essential amino acids isoleucine, leucine and valine, and inhibit plant growth by inactivating the target enzyme. The selective toxicity to weeds of these compounds or their analogs is due to their metabolism by particular crop species but not by most weed species. Thus, for those crop species sensitive to sulfonylureas and imidazolines, it would be of interest to develop crop hybrids or varieties having resistance to these herbicides.